As reported by Censor.NET citing BBC, in Brussels for a summit, she will say it is crucial that European countries work together to "protect our shared values and ideals". She will also discuss Brexit with European Council President Donald Tusk.
"From agriculture in Ukraine to the tech sector in Belarus - there is a huge amount of potential in the Eastern neighbourhood that we should nurture and develop.
"But we must also be open-eyed to the actions of hostile states like Russia which threaten this potential and attempt to tear our collective strength apart.
"This summit highlights the crucial importance of the European countries working together to protect our shared values and ideals.
"The UK may be leaving the EU but we are not leaving Europe, and we are unconditionally committed to maintaining Europe's security."
Last week Mrs May accused Russian President Vladimir Putin's government of trying to "undermine free societies" and "sow discord in the West," the PM's speech reads.